Top Hat has never been used at such depths and pressures before. BP began testing it on Tuesday to see if it could contain the oil, a process that could take up to 48 hours. As oil builds up under the cap the pressure should rise; if it does not, that will suggest oil is still leaking. In a statement BP said that “its efficiency and ability to contain the oil and gas cannot be assured”.

As oil builds up under the cap the pressure should rise; if it does not, that will suggest oil is still leaking